This invention relates to a switching device suitable for operating a lamp by means of a pulse-shaped voltage V1, comprising
switching means for generating the pulse-shaped voltage V1 from an input voltage, V1 having an amplitude proportional to the input voltage,
a pulse width modulator for supplying control pulses to the switching means,
a first auxiliary circuit for generating a first signal from the pulse-shaped voltage V1, this first signal being proportional to an average value of the pulse-shaped voltage V1, and
a control device for making the first signal substantially equal to a second signal through the supply of a control signal to the pulse width modulator.
Such a switching device is described in U.S. Pat. No. 4,156,166. The switching device described therein compares the first signal with a second signal which is independent of the supply voltage and the condition of the lamp. The difference between these two signals is amplified, which yields the control signal supplied to the pulse width modulator.
The aim of the known switching device is to achieve a substantially constant luminous flux of the operated lamp.
The luminous flux of an incandescent lamp depends on the effective value of the voltage with which the lamp is supplied. If the lamp is supplied, for example, from a DC-voltage source, like a single cell or a battery, the voltage of which decreases in proportion to the length of time the source has been in use, the luminous flux of the lamp will decrease accordingly.
If, however, a switching device as described in the opening paragraph, supplied with an input voltage E, is used for generating a pulse-shaped voltage V1, the amplitude of which is proportional to E, and which has a duty cycle D, it is possible to have D increase to such an extent, when E decreases, that the effective value of V1 remains substantially constant. The limit of the control range of the switching device is not reached until the moment D has increased to 100%.
If an incandescent lamp is supplied by means of such a pulse-shaped voltage, the effective value of which is substantially constant, the luminous flux supplied by the lamp will also be substantially constant. A disadvantage of the known switching devices, however, is that the effective value of the pulse-shaped voltage remains constant only to a first approximation, the deviation increasing in proportion as the change in the value of the input voltage increases. An object of the invention is for its object to indicate how the switching device can be realized so that the effective value of the pulse-shaped voltage generated by the switching device remains substantially constant, even with wide variations of the input voltage.